The secondary battery technology has been developed well, and thus the electric vehicle and the electronic handheld or mobile device can use the battery sets which can be charged by an external power source. In the conventional two stage charging method, the battery cells can be serially charged in a first stage (i.e. first stage charging). In such scheme, the battery cells are connected in a serial fashion, and a positive terminal of the first battery cell and a negative terminal of the last battery cell are respectively connected to an anode terminal and a cathode of the external power source, so as to perform the first stage charging.
After the first stage charging has been finished, a second stage charging is performed for separately charging the battery cells. In the second stage charging, a battery cell with a minimum cell voltage among the battery cells is selected to be charged to a saturation voltage. After the selected battery cell is charged to the saturation voltage, other one battery cell with the minimum cell voltage among the battery cells is selected to be charged to the saturation voltage. That is, all battery cells are charged to the saturation voltage in separate time period.
However, if the battery cell is charged long, a temperature value of the battery cell will increase. Due to chemical reaction and a current passing through an internal resistor of the battery cell, thermal energy is generated. In short, the temperature value of the battery cell increases when the battery cell is charged, and the battery cell may be damaged (i.e. lifetime may be decreased) if the battery cell is charged too long. Moreover, in the conventional two stage charging method, balancing for the battery cells is performed after all battery cells have been completely charged.